The elderly population experiences a high incidence of femoral fractures, commonly in the femoral neck and intertrochanteric regions. These fractures are normally treated by inserting a nail or hip screw from the side of the femur, through the neck, and into the femoral head. The nail or screw is then fixed to a side plate, that is fastened to the outside of the femur shaft, or to an intramedullary nail, that is inserted through the femur shaft. Both the side plate and the intramedullary nail may by secured to the femur shaft with bone screws.
A high incidence of death, between 10% to 25%, is associated with this type of hip fractures due to the injury itself or related complications. Frequent complications may arise when two or more bone fragments are forced towards each other when the patient supports his or her weight on the healing bone. For example, a sharp implanted nail or hip screw may cut through and penetrate the femoral head or neck; or a nail, hip screw, side plate, or intramedullary nail may bend or break under load where the contact between bone fragments is insufficient for the bone itself to carry the patient's weight.
Collapsible implants have been developed to maximize bone to bone contact by permitting bone fragments to migrate towards one another. Examples in the prior art include the Richards-type compression hip-screw and the Kenn-type nail. Richards screws comprise a long, smooth shaft and external threads at the tip. Kenn nails comprise a wide, tri-flanged tip at the end of a smooth shaft. In both examples, the nail or screw implanted through the neck of the femur is allowed to slide back through the side plate or intramedullary nail as the bone fragments move together under a load.
On the other hand, these known implants are laterally stiff. Their sharp ends may cut sideways through the cancellous tissue of a femoral head after implantation and migrate within the bone, either piercing the surface of the femur or simply no longer retaining proper alignment of bone fragments. To resolve this problem, single, helical blades were developed, such as the SPIRAL BLADE brand, currently sold by Synthes, Paoil, Pa., and such as disclosed in U.S. Pat. Nos. 5,300,074 and 4,978,349. These blades are twisted about 90.degree. along their length and have a substantially uniform width. When implanted into the neck and head of a femur, the distal end of the blade lies in parallel with the femur shaft, and the proximal end lies perpendicularly to the shaft. In this position, the load on the head acts on relatively flexible, large, flat surface, reducing the pressure on the cancellous tissue and diminishing the tendency of the implant to further cut through the bone once implanted. The distal end, being aligned with the femoral shaft, provides a higher bending stiffness than the tip to sufficiently support the blade. Also, unlike previous nails and screws, these blades require little or no material removal in the femoral head, prior to implantation, where the amount of bone is critical.
These single, helical blades, however, are fairly compliant in the transverse direction, towards the fore and aft of the patient's body, because of the vertical positioning of the distal end of the blade. Moreover, the blades provide little resistance to cutting through the cancellous bone like a knife in directions aligned with the width of the blade at any station along its length. A need exists, therefore, to provide improved osteosynthetic implants which do not have a tendency to cause such cutting.